1. Field of the Invention
This invention relates to devices containing liquid crystal oligomeric siloxane materials.
2. Background Information
Polymeric liquid crystals consisting of a siloxane backbone and a mesogenic portion are known, see for example E.P. 029 162 which discloses the preparation of such polymers by the addition of a polysiloxane having silicon-bonded hydrogen atoms to vinyl-substituted mesogenic molecules. More recently chiral smectic liquid crystal materials having ferroelectric properties have been described, see for example JP 01144491 which discloses materials containing ##STR4## in which Me represents methyl, R represents an optically active alkyl or aralkyl group, m=0 to 5 and n=2 to 10.
Desirable properties of ferroelectric liquid crystals include a low viscosity and a relatively high tilt angle which is also substantially independent of temperature. Low viscosities are related to reduced response times and are advantageous when a dye is to be incorporated into the liquid crystal material. A high tilt angle is advantageous with regard to the electro-optic properties of the liquid crystal. In a device based on birefringence effects the tilt angle is normally 22.5.degree. whereas devices incorporating dyes require an angle greater than this for good optical contrast. Measurements of the tilt angle of liquid crystal materials of the kind disclosed in JP 01144491 have shown not only a variation from polymer to polymer but also a reduction of the tilt angle with increasing temperature.
We have now surprisingly discovered that a certain narrowly defined group of oligomeric liquid crystal materials exist as neat compounds or in mixtures which exhibit relatively high tilt angles and that such angles are substantially independent of temperature over a broad temperature range including ambient.